The present invention relates to an apparatus and method which can be used to provide a seal between two portions of a semiconductor processing reactor which are operated at different pressures. The apparatus and method are particularly useful when the processing reactor operates over a broad temperature range (about 600.degree. C.) and the seal must bridge two materials having a substantially different coefficient of expansion. 2. Description of the Background Art
In the fabrication of electronic components such as semiconductor devices, the manufacturing process frequently requires that a substrate be cooled while that substrate is exposed to a partial pressure of about 10.sup.-3 Torr or lower. Processes which require substrate cooling under such partial vacuum conditions include, for example, physical vapor deposition (PVD), ion injection and particular forms of plasma etching.
PVD is used to deposit a thin film on a substrate. Films of materials such as, for example, aluminum, titanium, tungsten, tantalum, tantalum nitride, cobalt, and silica may be deposited on ceramic, glass or silicon-derived substrates using PVD processes such as a sputtering process. In a typical sputtering process, a low pressure atmosphere of an ionizable gas such as argon or helium is produced in a vacuum chamber. The pressure in the vacuum chamber is reduced to about 10.sup.-6 to 10.sup.-10 Torr, after which argon, for example, is introduced to produce an argon partial pressure ranging between about 0.0001 Torr (0.1 mTorr) and about 0.020 Torr (20 mTorr). Two electrodes, a cathode and an anode, are generally disposed in the vacuum chamber. The cathode is typically made of the material to be deposited or sputtered, and the anode is generally formed by the enclosure (particular walls of the vacuum chamber, or the platform upon which the substrate sits, for example). At times, an auxiliary anode may be used or the article to be coated may serve as the anode. A high voltage is applied between these two electrodes, and the substrate to be coated is disposed upon a platform positioned opposite the cathode. The platform upon which the substrate sets is often heated and/or cooled, and heat is transferred between the platform and the substrate, to assist in obtaining a smooth, even thin film coating upon the substrate. To obtain a smooth, even film coating, it is desirable to maintain the substrate at a uniform temperature within a few .degree.C.; preferably, the temperature is near but below the melting point of the material from which the film is being formed. It is very important that the substrate temperature be repeatable each time a given process is carried out. Thus, the heat transfer between the platform and the substrate must be uniform and repeatable.
When the pressure in the process (vacuum) chamber is about 1 Torr or less, convective/conductive heat transfer becomes impractical. This low pressure environment affects heat transfer between the substrate and the support platform; it also affects heat transfer between the support platform and heat transfer means such as a heating or cooling coil used adjacent to the platform to heat or cool the platform.
Since the substrate and the platform typically do not have the perfectly level surfaces which would enable sufficiently even heat transfer by direct conduction, it is helpful to provide a heat transfer fluid between the platform and the substrate, to assist in providing even heat transfer between the support platform and the substrate. It is known in the art to use one of the gases present in a PVD sputtering process as a heat transfer fluid between the support platform and the substrate. The fluid is typically a gas such as helium, argon, hydrogen, carbon tetrafluoride, or hexafluoroethane, for example, or other suitable gas that is a good heat conductor at low pressure. The fluid is generally applied through multiple openings or into exposed channels in the substrate-facing surface of the support platform. Presence of the heat transfer fluid between the substrate and support platform surface establishes a nearly static gas pressure which commonly ranges from about 1 Torr to about 100 Torr, depending on the particular film deposition process.
The positive pressure created by the heat transtar fluid used between the back (non-processed) side of the substrate and its support platform, as described above, tends to bow a thin substrate which is mechanically clamped at its edge. Bowing of the substrate in excess, for example 10 micrometers at the center of a typical silicon wafer substrate, was observed when the periphery of the substrate was held by mechanical clamps. This substrate bowing reduces the amount of heat transfer near the center of the substrate and results in uneven heating of the substrate in general. Further, the gas used to provide a heat transfer fluid between the substrate and the support platform leaks from the edges of the substrate so that a constant net flow of fluid occurs from beneath the substrate into the process vacuum chamber. The amount of gas leakage commonly ranges from about 10% to about 30% of the gas used during processing of the substrate.
One means of avoiding bowing of the substrate and of reducing heat transfer fluid leakage at the substrate edge is the use of an electrostatic chuck as the support platform. An electrostatic chuck secures the entire lower surface of a substrate by Coulombic force and provides an alternative to mechanical clamping of the substrate to the support platform. When a substrate is secured to the platform using an electrostatic chuck, the flatness of the substrate during processing is improved. In a typical electrostatic chuck, the substrate (comprised of a semiconductor material or a non-magnetic, electrically conductive material) effectively forms a first plate of a parallel-plate capacitor. The remainder of the capacitor is generally formed by the substrate support platform which typically comprises a dielectric layer positioned on the upper surface of a second conductive plate.
The support platform upon which the substrate sets can be heated or cooled in a variety of manners, depending on the kind of process to be carried out in the process chamber. For example, radiant, inductive, or resistance heating can be used to heat a support platform; the platform can be heated or cooled using a heat transfer fluid which is circulated through internal passageways within the support platform; in the alternative, heating or cooling can be achieved using a heat transfer surface such as a heating or cooling coil which is located adjacent to, frequently in contact with, the support platform. The means of heating or cooling the support platform often depends on the materials of construction of the platform itself. When the partial pressure in the process chamber is less than about 1 Torr, the support platform cannot be heated or cooled using a heat transfer surface adjacent the support platform, since convection/conduction of heat cannot occur at a practical rate.
There are numerous materials of construction and structural possibilities described in the art which can be used to form an electrostatic chuck. The means for heating and cooling the electrostatic chuck (from which the substrate can be heated or cooled) depends on the materials of construction used and the overall structure of the electrostatic chuck.
U.S. Pat. No. 4,771,730 to Masashi Tezuka, issued Sep. 20, 1988 describes an electrostatic chuck comprised of a conductive specimen table (probably constructed from a metallic material such as aluminum) having hollow conduits therein for the circulation of water. Mounted to the upper surface of the specimen table is an electrode comprising an electrode plate constructed of a metal such as aluminum, surrounded by a dielectric film of a material such as Al.sub.2 O.sub.3 (alumina). The substrate to be processed sets upon the upper surface of the dielectric film material. It is readily apparent that the sandwiched materials of construction which make up the electrostatic chuck have vastly different coefficients of expansion. Upon exposure of the sandwiched materials to operational temperature ranges of several hundred degrees Centigrade, stress is created between the sandwiched layers which can lead to deformation or fracture of the more fragile layers and to poor performance and eventual failure of the electrostatic chuck in general.
U.S. Pat. No. 5,155,652 to Logan et al., issued Oct. 13, 1992, discloses an electrostatic chuck assembly including, from top to bottom: a top isolation layer; an electrostatic pattern layer comprised of an electrically conductive electrostatic pattern disposed on a substrate; a heating layer comprised of an electrically conductive heating pattern disposed on a substrate; a support layer; and a heat sink base having backside cooling and insulating channels provided therein. The preferred material for the isolation layer is Boralloy 11.RTM., a pyrolytic boron nitride available from Union Carbide. The electrostatic pattern layer is not particularly defined. The heating layer is comprised of a substrate, preferably pyrolytic boron nitride having a conductive heating pattern disposed thereon. The conductive heating pattern is preferably comprised of a pyrolyric graphite. The support layer is preferably comprised of a boron nitride having metal vias disposed therethrough for conducting electrical energy to metal vias within the hearing layer. The heat sink base is comprised of a thermally conductive block of material having clearance holes extending therethrough for facilitating electrical contact with the metal vias of the support layer. The heat sink base also has channels therein for the circulation of a cooling fluid. The material of selection for heat sink base is said to be critical because it must match the thermal expansion rate of all the other layers in the structure. It is recommended that KOVAR.RTM., an iron/nickel/cobalt alloy available from Westinghouse Electric Co. be used to form the heat sink base.
The heat ink base is said to be bonded to the bottom of the support layer using one of several techniques. The techniques include brazing, whereby gold contact pads are deposited on the respective bonding surfaces, the pieces are fitted together, and the assembly is heated in a brazing furnace. A second method of bonding the heat sink base to the bottom of the support layer is to apply a thermally conductive ceramic cement. A third method is to mechanically clamp the two pieces together by fabricating a flange on the bottom of the support layer and a clamp ring on the top of the heat sink base.
The kind of structure described in the Logan et al. is very expensive to construct, both in terms of materials of construction and fabrication of the structure itself.
U.S. Pat. No. 5,191,506 to Logan et al., issued Mar. 2, 1993 describes an electrostatic chuck assembly including, from top to bottom: a top multilayer ceramic insulating layer; an electrostatic pattern layer having a conductive electrostatic pattern disposed upon a multilayer ceramic substrate; a multilayer ceramic support layer; and a heat sink base having backside cooling channels machined therein. The multilayer ceramic structures are bonded together using known techniques applicable to multilayer ceramics, and the heatsink base is brazed to the bottom of the multilayer ceramic support layer. The materials of construction are essentially equivalent to the materials of construction described in U.S. Pat. No. 5,155,652. The heat sink base is said to be brazed to the electrostatic pattern layer by depositing gold contact pads on the respective bonding surfaces, fitting the pieces together, and heating the assembly in a brazing furnace.
In the above-referenced U.S. patents, when cooling is involved, the cooling is accomplished by using a fluid flowing: a) in direct contact with either a specialized metal alloy having a coefficient of expansion matched to that of a dielectric material with which it was in contact, or b) in direct contact with the dielectric material itself. This is necessary since the entire vacuum chamber in which substrate processing is carried out is operated at a partial vacuum which renders conductive/convective heat transfer impractical. The use a specialized metal alloy heatsink base of the kind described in the above-referenced patents is very expensive. Further, direct contact of the cooling fluid with the kinds of dielectric materials generally described may not provide effective heat transfer and may result in fracture of the dielectric material itself. The ceramic materials typically used as dielectrics are commonly sensitive to temperature differential, and the temperature differential between a heating element and a nearby cooling element can cause the ceramic to fracture.
It would be very advantageous to have a means for cooling an electrostatic chuck (or any other substrate support platform used in low pressure semiconductor processing) which did not require the use of such expensive materials of construction and which provided an efficient means of heat transfer.